JP-A-6-295209 discloses a flow control valve.
Referring to FIG. 5, the flow control valve disclosed in JP-A-6-295209 includes: a main body 53 having an inlet passage 51 and outlet passage 52; a valve portion 54; and a valve element 57 having a first diaphragm portion 55 and second diaphragm portion 56. By the first diaphragm portion 55 and second diaphragm portion 56, the chamber of the main body 53 is divided into a first pressure chamber 58 and a second pressure chamber 59. Constant pressure directed inward is given at all times from the outside of the first pressure chamber 59 by utilizing compressed air. Further, constant pressure directed inward is given at all times by the spring 60 provided inside the second pressure chamber 59. The pressure receiving area of the first diaphragm portion 55 is larger than the pressure receiving area of the second diaphragm portion 56.
However, the valve device, in which the above diaphragms are used, is disadvantageous in that gas is transmitted through the diaphragms. Especially, in the case of a pipe line in which chemicals are used, corrosive gas is generated, by the chemicals, in some cases. It is well known that the thus generated corrosive gas is transmitted through the diaphragms and metallic parts arranged close to the diaphragms are corroded.
In the case of the flow control valve disclosed in the above official gazette, the spring 60 is arranged on a lower face of the second diaphragm portion 56. Therefore, in order to protect the spring 60 from the corrosive gas which has been transmitted through the second diaphragm portion 56, it is necessary to conduct coating on the spring 60 with PTFE and others. However, when coating is conducted on the spring 60, the spring constant of the spring 60 is changed by the coating thickness, which could be one of the reasons by which a difference is caused between the individual flow control valves.
In general, in the flow control valve, it is well known that control can be precisely conducted and the control performance can be enhanced when a change in the degree of opening of the valve is small with respect to a traveling distance of the valve element. In the case of the flow control valve disclosed in the above official gazette, when a change in the opening area of the hydraulic control passage is small with respect to a traveling distance of the valve element 57 in the axial direction, the flow control performance is enhanced. In order to reduce the change in the opening area with respect to the traveling distance of the valve element 57, it is necessary to design a valve portion 54 the diameter of which is small. Accordingly, it is necessary to design a rod, the diameter of which is small, for connecting the valve portion 54 with the first diaphragm portion 55. Further, it is necessary to design a rod, the diameter of which is small, for connecting the valve portion 54 with the second diaphragm portion 56.
In the flow control valve disclosed in the above official gazette, when the position of the valve 54 is adjusted by the pressure in the first pressure chamber 58, the opening area of the flow rate control passage 61 can be adjusted. On the other hand, whether or not the diaphragm portion is liable to be deformed depends upon the diameter of the diaphragm portion. Accordingly, the larger the diameter of the diaphragm is, the more linearly the pressure in the first pressure chamber 58 is changed with respect to the displacement of the valve portion 54 and the lower the hysteresis becomes. On the other hand, when the diameters of the first diaphragm portion 55 and the second diaphragm portion 56 are decreased, it becomes difficult for these diaphragm portions to be deformed, and the change in the pressure in the first pressure chamber 58 with respect to the displacement of the valve portion 54 becomes non-linear, and the hysteresis is increased.
That is, in order to enhance the hydraulic control performance of the flow control valve, it is preferable that the diameter of the valve portion 54 is decreased and the diameter of the diaphragm portion is increased.
In this case, when the diameter of the first diaphragm portion 55 is increased and the diameter of the valve portion 54 is decreased, the intensity of the force, which is caused by the pressure in the first pressure chamber 58 and pushes the first diaphragm portion 55 downward, is increased, and the intensity of the force, which is caused by the pressure of the downstream side fluid and pushes the first diaphragm portion 55 upward, is also increased. Therefore, in order to keep the balance of the force, the intensity of the upward force generated by a repulsive force of the spring must be increased.
At this time, these forces are received by a rod for connecting the upper and lower end faces of the valve portion 54 with the first diaphragm portion 55 and the second diaphragm portion 56. This rod is compressed in the axial direction at all times. Therefore, it is necessary to give attention to the mechanical strength of the rod. Especially when the hydraulic fluid is a corrosive fluid of high temperature, the rod is usually made of PTFE, the chemicals-resistance-property of which is high. However, PTFE is characterized in that the mechanical strength is low and, further, PTFE creeps easily. Accordingly, when the rod made of PTFE is used over a long period of time, it is deformed or buckled, and the hydraulic control performance of the flow control valve is deteriorated.